In a finding that won't surprise many mothers, a new study says breast-feeding may help secure the bond between mother and child. But the study also offers one explanation how: through a change in the mother's brain.

The brains of breast-feeding mothers show a greater response to the sound of their babies' cries than do the brains of mothers who do not breast-feed, the study researchers say.

This boost in brain activity is seen in brain regions associated with mothering behaviors.

The finding adds to a growing list of the benefits of breast-feeding. Breast milk is considered the best source of nutrition for babies, and breast-feeding has been linked with better test scores and better health for the child later in life.

The results suggest this brain activity facilitates greater sensitivity from the mother toward her infant as the baby begins to socially interact with the world, the researchers say.

The study may help people to "recognize that it's important to support mothers who do want to breast-feed," said study researcher Pilyoung Kim, of the National Institute of Mental Health.

That's not to say that women must breast-feed. Some women choose not to breast-feed, while others can't, either because of biological problems or other issues, including constraints imposed on them by their jobs. Kim herself has a 1-year old son and has had difficulties with breast-feeding.

"I understand the challenges mothers have," Kim said. "Regardless of their decision, I think it is critical during this early postpartum period that they seek support and encouragement from others, especially when they feel very stressed and challenged by the new demands because of the new parenting experience."

An infant's cry

Kim and her colleagues examined 17 new mothers. Nine of the mothers breast-fed while the other eight used formula to feed their babies.

Two to four weeks after giving birth, the mothers had their brains scanned using a functional magnetic resonance (fMRI) machine while they listened to recordings of both their own baby's cries and the cries of newborns who weren't their children.

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Mothers who breast-fed showed greater activity in several brain regions, including the superior frontal gyrus, striatum and amygdala. Studies on animals have found links between these brain regions and parenting behavior.

The researchers also examined the mothers' behavior in the home. The women were videotaped interacting with their 3- to 4-week-old infants. The researchers rated the mothers on how affectionate, or sensitive, they were toward their babies. The ratings were based on factors such has returning a smile to the infant or responding appropriately when the infant was stressed, Kim said.

Regardless of whether the mothers breast-fed or formula-fed their babies, increased activity in the mothers' superior frontal gyrus and amygdala was associated with greater maternal sensitivity, the researchers say.

Developing empathy

The brain regions activated in the study may be responsible for empathy. So high activity in these regions may contribute to the breast-feeding mother's ability to understand how her own infant is feeling and respond in an appropriate way, the researchers say.

These brain regions are "definitely doing something to help process the information and perhaps motivate the mothers to exhibit more caregiving behaviors," Kim said.

It's possible hormones released during breast-feeding, such as oxytocin, may contribute to brain and behavioral changes in the mother. Research is needed on larger groups of people to better understand the relationship between breast-feeding and brain responses, Kim said.

A better understanding of this relationship may help researchers learn why some mothers have trouble forming an emotional bond with their child, and perhaps lead to a treatment or intervention for those mothers, Kim said.

The study was published online April 18 in the Journal of Child Psychology and Psychiatry.

Pass it on: Breast-feeding mothers show a greater response in their brains to their own infants' cry than do non-breast-feeding mothers.